(1) Field of the Invention
The present invention generally relates to a direct conversion radio frequency (RF) receiver, or a homodyne RF receiver, and particularly relates to a mixer for the homodyne RF receiver.
(2) Description of the Prior Art
Traditional Radio Frequency (RF) products usually employ a heterodyne RF receiver to receiving RF signal. In conventional wireless communication products, receivers usually utilize heterodyne technique, which is remarkable for its performance. Other kinds of receivers, such as the direct conversion RF receiver, the wideband IF receiver or the low IF receiver may be referred to deformed techniques of the heterodyne RF receiver.
The heterodyne RF receiver requires not only costly discrete devices but also application of external signal conversion. Heterodyne receivers convert RF signals from all channels into intermediate frequency (IF) signals by means of an external signal filter, and apply the IF signals to a local OSC and an external Voltage Control Oscillator (VCO) for conversion to base band signals, raising costs and limiting yield.
Therefore, direct conversion techniques, with lower power consumption and better suitability for multimedia systems, are widely used in receivers, omitting the need for IF signals conversion. The direct conversion RF receiver, also called a homodyne RF receiver, has another advantage of the system on a chip (SoC) application. The homodyne RF receiver can be regarded as a simplified heterodyne RF receiver, which has a zero intermediate frequency. So it also called a Zero IF receiver.
Please refer to FIG. 1. It shows a functional block diagram according to typical homodyne RF receiver. A typical homodyne RF receiver 10 at least comprises a LNA 14, a mixer (16a or 16b), a baseband amplifier (22a or 22b), a low pass filter (23a or 23b), an analog/digital convertor 24 and a DSP 26. The homodyne RF receiver 10 can be separated into a I channel and a Q channel. The mixer 16a, the baseband amplifier 22a, the low pass filter 23a and the ADC 24a are belonging to the I channel. The other set of the same elements (16b, 22b, 23b and 24b) is belonging to the Q channel.
In some prior arts, there could be a pre-selection filter 12 before the LNA 14 to filter signal from the antenna, here, predetermined out-of-band signals would be filtered out. Sometimes, the pre-selection filter 12 also has the function of eliminating the image frequencies. The output point of the pre-selection filter 12 is coupled with the LNA 14. For example, according to the specification of IEEE 802.11b, the received RF signal, the pre-selection filter 12 and the LNA 14 are all operated in a frequency between 2.4 GHz to 2.48 GHz. The output point of LNA 14 is coupled with the mixers 16a and 16b, individually. A local oscillator 18 provides LO signals. A frequency divider 15 generates phase difference of the LO signal for the I channel and the Q channel. Take the wireless specification of IEEE 802.11b for example, the local oscillator 18 is operated under a frequency of 2.4 GHz, to convert the signal to a low frequency signal nearby DC level.
Since the homodyne RF receiver direct down-converts the signal to nearby DC, performance of the mixer (16a or 16b) is more sensitive to LO self-mixing and low frequency performance. In the mixer (16a or 16b), as the desired signal converts to nearby DC, a folding load stage is implemented to adjust DC level for subsequent stage (i.e. the baseband amplifier 22a, the low pass filter 23a and the ADC 24a . . . etc.). Mentioned folding load stage is implemented after the gain stage and the switch stage of the mixer. However, it places additional challenges to IIP3 (third order input intercept point) performance and noise figure performance.